A construction machine such as a hydraulic excavator employs fuel (gasoline, light oil, etc.) as the power source of its engine and drives hydraulic actuators (hydraulic motor, hydraulic cylinder, etc.) using hydraulic pressure generated by a hydraulic pump which is driven by the engine. Being small-sized, lightweight and capable of outputting high power, the hydraulic actuators are widely used as actuators for a construction machine.
Meanwhile, there has recently been proposed a construction machine employing an electric motor and an electrical storage device (battery, electric double layer capacitor, etc.) and thereby realizing higher energy efficiency and more energy saving compared to a conventional construction machine employing hydraulic actuators only (see Patent Document 1).
Electric motors (electric actuators) have some excellent features in terms of energy, such as higher energy efficiency compared to hydraulic actuators and the ability to regenerate electric energy from kinetic energy at the time of braking. The kinetic energy is released and lost as heat in the case of hydraulic actuators.
For example, the Patent Document 1 discloses an embodiment of a hydraulic excavator having an electric motor as the actuator for driving the swing structure. The actuator for driving and rotating the upper swing structure of the hydraulic excavator with respect to the lower track structure (implemented by a hydraulic motor in conventional hydraulic excavators) is used frequently and repeats activation/stoppage and acceleration/deceleration frequently in work.
When a hydraulic actuator is used for driving the swing structure, the kinetic energy of the swing structure in deceleration (braking) is lost as heat in the hydraulic circuit. In contrast, energy saving can be realized by use of an electric motor since regeneration of the kinetic energy into electric energy is possible.
There has also been proposed a construction machine that is equipped with both a hydraulic motor and an electric motor so as to drive the swing structure by total torque of the hydraulic motor and the electric motor (see Patent Documents 2 and 3).
The Patent Document 2 discloses an energy regeneration device of a hydraulic construction machine in which an electric motor is connected directly to the hydraulic motor for driving the swing structure. A controller determines the output torque of the electric motor based on the operation amount of the operating lever and sends an output torque command to the electric motor. In deceleration (braking), the electric motor regenerates the kinetic energy of the swing structure into electric energy and accumulates the regenerated energy in a battery.
The Patent Document 3 discloses a hybrid construction machine which performs output torque splitting between the hydraulic motor and the electric motor by calculating a torque command value for the electric motor using the differential pressure between the inlet side and the outlet side of the hydraulic motor for the swing driving.
Both of the conventional techniques of the Patent Documents 2 and 3 employ an electric motor and a hydraulic motor together as the actuators for the swing driving and thereby realize operation with no feeling of strangeness even for operators accustomed to a conventional construction machine driven by a hydraulic actuator, as well as achieving energy saving with a configuration that is simple and easy to put into practical use.